Radio protective device



Feb- 8, 1949. L. M. HOLLINGSWORTH ET AL 2,461,153

RADIO PROTECTIVE DEVICE Filed Feb. 8, @45

EECE/I/EE LOWELL MHOLLM/GSM/ORTH 'LO U/.S` D. SMULL/N to protect the crystal.

Patented Feb. 8, 1949 STATES PATENT OFFICE,

2,461,158 RADIO PROTECTIVE DEVICE.

VLowellv VM. Hollings-vvorth, Cambridge, and'. Louis D. Smuliin, Arlington, Mass.,y assignors, by 'mesne assignments, to the United States of America as represented by the Secretary of War Application 'February s, 19451, serial No. 576;843

5 Claims. 1.

The Lpresent invention relates to radio objectlocati-ng systems, adapted to transmit and re- 'ceivepulses'oi high lfrequency energy, and it is particularly directed to a device for protecting ceiyer componentA of such a system from -llgh tehs'ity pulses.

Vasystem of Athel above type, whenever a single antenna is used for both transmitting an vingthe problemarises of insuring that 'm usei's made o1" the available energy. @ne Soi ion is to provide a transmit-receive or iF-Ris'wltch to transfer the antenna connection from the 'i'ecei-ver to the transmitter for the 'du 'tion of the transmitted signal pulses. An rl portant-function of the switch is the protection or tlfiereceiver input circuit. At frequencies where RF amplifiers may be used, the-amplifier tube inthe input stage. can 'be chosen to -wi`thstand large input powers without damage. Howover, at microwave frequencies, the input stage includes a mixer which may be easily damaged 'by `large applied signals and so must be' more carefully protected. This is particularly true of crystal 'mixers which are extremely delicate.

When the power of a radio object-locating set is onfthe delicate crystal in the receiver circuit is. lirlsuall'y -protected from large doses of RF energnfforiexam'ple, by the spark-'gap of a cavity type switch which breaks down when it is overloaded, thus shorting out the receiver. As iswellr-known, a keep-alive voltage is applied acr'ossflthefgap to maintain a glow discharge to piovideions for quickly Vforming an arc across thefsparksgapl when the transmitted pulse is applied; When 'the power is off, however, the keeps-alive. voltage vacross the gap is off, the

inethe TR cavity is not ionized, and breakdown Aof the gap vcannot occur rapidly enough Therefore, when the keepealive of 'the vset is not on, the receiver crystal is susceptible to damage, particularly f-rmalarge'output `signals of nearby radio objectlocating sets. There is also danger that the cnysta-lfmay be damaged as a result of mislring Giobbe-'magnetron of the transmitting apparatus on starting.

Itis one ofthe objects of the present invention to=fprovide means for protecting the receiver circuitoffa `radioA object-locating apparatus from overload 'when the keep-alive of a TR switch is; not incineration.

lt'is'another object of the invention to providea mechanical device which is adapted to preventlargeRF ltransmitted l pulses from `entering-thereceiver circuit;`

'tl isfstill another v'object of this invention to provide, in associationfvvith a TR cavity coupled by means of.v van -iris rto a `receiver transmission line, am'ovable mechanical shutter or shield: -for protecting the receiver crystal by shielding. --the iris from the cavity reld and by snorting-out coupling currents.

Other objects and advantages of the `rinvention will become apparent as thev description proceeds, reference being made vtothe"dra-wings in: which:

Fig.` i is atop plan view oi a 'IReaVity coupled to fa waveguide and to `a receiver transmission line and-embodying the device accordingV to v'the present. invention;

2 is a sectionalv View along the `line f2'2 of Fig. 1, and

Fig. 3 is a sectional view, partly brokenfaway, `along the line `3erof Fig. 2.

A ypreferred Y`embodiment of the invention is illustrated 'in the drawings in which ty indicates a cavity type' TR boXor switch of conventional design, having a spark-.gap formed by two; opposing electrodes generally `indicated at H mountedonthe-lend' walls l2 and t3 of l-b'ox 10. Box t!) is coupled in any `suitable manner/to' a transmission line '14 'connecting a transmitter apparatus 40 to-an antenna '42 and to a trans'- mission =line vl5 connecting to a receiver 44. Lines l-l andl5 are shown preferably asf-a'wave guide and va ycoaxial line respectively, suitably coupled to box t0, 'for example, as shown', by meansof irisesor apertures t6' and Vl'l.

Disposed in side Wall 'I8 of 'box I0 and near the Coupnng'toune-ls isaiongitudinai.S10-1; rer

having its longitudinal -a'xis substantially perpendicular'to end IWalls I2 Aand I3. vSlot I'S -is relativelyn'arrow in lwidth and has a length-sub`- stantially equal to the width of side wall lll-for permitting entry of 'a shutter member 20 into the cavity dened by box l0. Shutter member 2|) may be a flat rectangularly shaped'stripbut asshown -is v.preferably a curvedl or a-rc'iuate` strip or sheetfof metal or othersuitable electrical conducting material .preferably shaped as a portion of a cylindrical surface. Member y20-is mounted in'any suitable manner so that it maybe moved into and' out of the cavity of box lll'through slot t9. For example, member 20 may be `rigidly mounted'on-fan arm -2l to rotate vtherewith 'about a piv'ot 22. Member 20v may beactuatedl inany desired manner, vsuch 'asby 1a solenoid `23 through. lever mechanism- Vgenerally lindicated at -24. The solenoid -andrlever mechanism are so arranged that when -powersource tu is turned theshutter member 2, which is normally in the position indicated by dotted lines in Fig. 3 when the system is in operation, is adapted to be automatically swung into box Il) through slot i9 and into position in front of the iris II. Also the shutter member 2! may be adapted to be automatically withdrawn from the cavity when the power is turned on. However it has been found desirable to provide a separate control for selectively operating the solenoid or other actuating means when the power of the set is turned on whereby the shutter 2S may be moved out of box Ii) or maintained in position to shield the receiver apparatus against damage such as may be caused by misring of the magnetron of the transmitting apparatus on starting.

It should be noted that in addition to shielding the coupling aperture i1 from the fields in the cavity of box Il), shutter member 20 Should be brought into good electrical contact with the transmission line l5, or as shown the outer conductor of the coaxial line I5, so that current-s that would ordinarily induce voltages in the iris coupling Il are adapted to be shorted-out through shutter member 2B. To accomplish this shutter member 20 is preferably moved a suiicient distance into box Iii to be in front of iris I1 and in slidable but rrn contact with the wall portions of transmission line I adjacent to iris I'I. Similarly, the short-circuiting action may be achieved by lshutter member making good contact with end walls I2 and i3 of box ID. Also shutter member 26 may make contact with the side wall i8 at slot Ie and by engagement or abutment of the free end of shutter 2U with the inside surface of side wall I8 or the far side of coupling I'I as at 25, to further insure good shielding action.

It will be understood that not only is shutter 2G adapted to protect the receiver by shielding the iris Ii from electromagnetic fields in the cavity and by shorting-out coupling currents, but the insertion of shutter member 20 also serves to detune the TR cavity.

While a preferred embodiment of the invention has been described above and illustrated in the drawings, it will be understood that the invention is capable of modification and improvement. For example, the shutter member 2c may be a flat plane member. Also, instead of being insertable through a slot in the side wall of the TR cavity, member 2! may be insertedthrough a straight or arcuate slot in one of the end walls I2 or I3, and if desired, slidably guided into place by means of grooves provided in the side wall I8 at points Ie and 25. Movement of this member 2e may be actuated by mechanical or electrical means; for example, by a relay or other tripping mechanism. Therefore, it is not desired that the present invention be limited to the precise details set forth.

What is claimed is:

l. A radio object-locating system operable from a source of electric power and comprising, in combination, an antenna for transmitting and receiving pulsed electromagnetic energy, transmitting apparatus electrically connected to said antenna thru a pair of transmission lines and receiving apparatus, wave transmission means including a walled cavity type transmit-receive switch electrically coupled to said transmission lines, the said switch being' adapted to afford operative connections from said antenna alternatively to said transmitting apparatus and to said receiving apparatus, said switch enabling said wave transmission means to pass electromagnetic energy from said transmitting apparatus to said antenna and having a portion thereof in which a coupling is normally effected between said antenna and said receiving apparatus except for the duration of energy pulse transmissions by said system, a sheet-like electrically conductive member arranged to be pivotally inserted through a suitable slot in the wall of said switch, means under control of said power source for moving said member and operative when said system is cut off from said power source to move said member into electrical contact with portions of said transmission means affording connection to said receiver apparatus adjacent said coupling portion for effecting cavity detuning and shortcircuiting action thereby to protect said receiving apparatus from energy fields within said switch.

2l The combination, as claimed in claim 1, wherein the transmit-receive switch comprises a cavity dened by opposing parallel plane end walls and a circular side wall perpendicular to said end walls, wherein the coupling portion comprises an iris coupling providing communication between said cavity and said transmission means connecting to said receiver apparatus, the slot being disposed in said side wall with its longitudinal axis perpendicular to said end walls, the length of said slot being substantially equal to the width of said side wall, and wherein the sheet-like member is shaped as a portion of a cylindrical surface having' a width substantially equal to the length of said slot and suicient to make contact with said end walls and a length sufficient to make good contact with said side wall at said slot and at the opposite side of said iris coupling.

3. The combination as claimed in claim 1 wherein the transmission means connecting the switch to the receiver apparatus is a co-axial transmission line, said coupling portion comprising an iris in the outer wall of said line providing communication between said line and the cavity of Said switch and wherein the sheet-like member is adapted to be moved into electrical contact with wall portions of said line adjacent said iris.

4. A switch of the transmit-receive walled cavity type for use in a radio object-locating system operable from a source of electric power having an antenna for transmitting and receiving pulsed electromagnetic energy, transmitting apparatus and receiving apparatus, comprising, in combination, means for coupling said cavity to said transmitting and receiving apparatus,v a wall portion of said switch having a slot, a member shaped as a portion of a cylindrical surface being insertable into said cavity through said slot into a position in front of said means coupling said cavity to said receiving apparatus, the curved edge portions of said member making good electrical contact with'the adjacent wall portions of said switch, and means for moving said member into said position whereby said member is adapted to shield said coupling for protecting said receiving apparatus from damage by large signal pulses.

5. A switch of the Vtransmit-receive walled cavity type for use in radio object-locating system operable from a source of electric power and having an antenna for transmitting and receiving pulsed electromagnetic energy connected" to a transmitting apparatus anda receiving apparatus through transmission means, said switch including means for coupling s aid switch to said transmission means, a wall por-tion of saidswltch having a slot, a sheet-like member insertable into the cavity of said switch through said slot into position to shield said last mentioned coupling means and into good electrical contact with portions of said transmission means connecting said switch to said receiving apparatus adjacent said last mentioned coupling means, and control means for moving said member into said position thereby protecting said receiving apparatus from damage by unwanted signal pulses, said control means being normally automatically operable when said apparatus is cut-off from said REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,396,D44 Fox Mar. 5, 1946 2,421,028 King May 27, 1947 

